As measurement technique of a minimal area, a scanning probe microscope (SPM) is known. Out of scanning probe microscopes, an atomic force microscope (AFM) is widely used for technique for scanning a surface of a sample by controlling a probe an end of which is pointed and enabling the measurement of surface profile in atomic size (refer to Physical Review Letters, vol. 56, No. 9, p. 930). However, in the atomic force microscope, an optical property such as the distribution of a refractive index on a surface of a sample cannot be measured.
In the meantime, in a most advanced minimal semiconductor device, the performance is enhanced by controlling physical properties in units of a nanometer and the physical properties except a shape are required so be measured in units of a nanometer. Besides, in a storage device and others, as a minimal foreign matter is fatal for the operation of the device, the detailed physical property of the foreign matter is required.
Optical spectral measurement is suitable for the measurement of a physical property, heretofore, Raman microscope for Raman spectroscopy and others are developed, and they are widely utilized for analysis. However, in conventional type optical microscope technique, spatial resolution is approximately a few hundreds nm and as the resolution is short for observation in units of nanometer, the details of a foreign matter cannot be observed.
To address these problems and to measure physical information and an optical property of a surface of a sample at high spatial resolution, there is provided a scanning near-field optical microscope (NSOM).
The scanning near-field microscope uses near-field light that leaks from a minute aperture in the size of approximately a few tens nm in means called an aperture probe for example as described in Chemical Reviews, 1999, vol. 99, No. 10, pp. 2891-2927. An optical property of a surface of a sample can be measured at spatial resolution of a few tens nm similar to the aperture by holding clearance between the minute aperture and the sample in a range of a few nm to a few tens nm and scanning the aperture.
Besides, in Optics Letters, vol. 19, no. 3, p. 159, a scanning near-field optical microscope that radiates light to a pointed end of a probe and realizes optical observation at spatial resolution of a few tens nm using near-field light generated at the end of the probe depending upon interaction with a sample and scattered light of the near-field light is also disclosed. This technique is known as a scattering probe.
In the scanning near-field optical microscope, light that leaks from the minute aperture is feeble according to the aperture probe for example and as interaction with the sample is weak in the scattering probe, detected light is feeble, high-precision and sensitive measurement is difficult.
Japanese Unexamined Patent Application Publication No. 1999-316240 discloses a means that intermittently excites near-field light excited at an end of a probe in a scattering probe and realizes sensitive measurement. However, in the technique, it is not settled that excited light itself is directly mixed with observed light and sensitive measurement is difficult. Further, as the excitation of the near-field light is intermittent, there is a problem that the absolute luminous energy used for measurement of near-field light is short and sensitive measurement is basically difficult.
As described above, to measure the optical property at the spatial resolution in units of nanometer, it is effective that the scanning near-field optical microscope is used. However, as described above, there is the problem that sensitive and high-precision measurement is very difficult.
To address this problem, a measurement method of plasmon propagation-type optical SPM disclosed in Journal of Applied Physics, vol. 109, no. 1, p. 013110 can be given. In the plasmon propagation-type optical SPM, light is radiated to a cantilever so as to excite plasmon and near-field light is generated at the forefront of a probe. As the near-field light excited at the forefront of the probe is scattered when the near-field light approaches a sample or is touched to the sample, measurement is made by observing scattered light as propagated light in a distant place. In this technique, as light is measured in synchronization with the oscillation of the cantilever, the scattered light is detected at a frequency of the oscillation of the cantilever.